National Geographic : 1968 Aug
Squad of synthetic sapphires glows in invisible ultraviolet light. Each has been impregnated with different metallic atoms such as manganese, cobalt, or chromium, causing the sapphires to fluoresce with varied colors under "black light." Lapidaries can distinguish synthetic gems from natural only under magnification or by chemical analysis; synthetic sapphires and rubies now abound in jewelry shops. Resembling taffy pulled from a pot, a silicon ingot grows by the "crystal-pulling" process at Texas Instruments Incorporated. A rod lowers a silicon crystal seed into a crucible of liquid silicon at 2,6000 F., slightly above its melting point. The cooler seed causes the melt to freeze, or crystallize, at the point of contact. Withdrawn as crystalli zation takes place, the crystal grows six inches an hour. Slow rotation, preventing growth along natural crystalline planes, gives it a cylindrical contour. Such crystals form a base for integrated circuits (pages 286-7). 284 Within recent years, vacuum tubes have become obsolete for many purposes. In appli cations that do not require much power, they have been replaced by the transistor, invented two decades ago at Bell Telephone Labora tories. The transistor, made from germanium or silicon-substances known as semicon ductors-performs the functions of the vac uum tube with a speck of crystal smaller than the letter "o" on this page. Transistors can be encased in metal cans roughly the size of a garden pea and soldered into a wired circuit. Markedly smaller and lighter than vacuum tubes, they require far less power and give off relatively little heat. And they have proved to be vastly more reli able and long-lived.